1. Field of the Invention
This invention relates to methods for increasing the rate of production of methane from a subterranean carbonaceous formation by chemically stimulating the formation with an aqueous oxidizing solution to increase the production rate of methane from the formation. The invention is applicable to the enhanced recovery of methane from formations consisting of carbonaceous materials deposited with inorganic materials, such as occur in carbonaceous shale formations. The increased production rate is accomplished by increasing the surface areas of the contained organic material fragments, which contain hydrocarbons, by inducing the formation of cleats and other new surfaces in these carbonaceous materials, thereby facilitating the desorbtion of light hydrocarbons from these carbonaceous formations. Carbonaceous formations such as shales, are composed in part of clay minerals. The invention is also applicable to the enhanced recovery of light hydrocarbons which are adsorbed to these clay minerals.
2. Brief Description of the Prior Art
Substantial quantities of methane gas are found in subterranean coal formations and in other formations containing carbonaceous materials, which may include macerals, kerogens, and other organic materials and which are present with inorganic materials such as sands, clays and like clastic materials in the formation. Such other formations are referred to herein as "carbonaceous formations".
A variety of processes have been used in attempts to recover the methane from such formations, especially coal formations, more efficiently.
The simplest process is the pressure reduction process wherein a borehole is drilled into a coal formation from the surface and methane is withdrawn from the borehole by reducing the pressure to cause methane to be desorbed from and flow from the coal formation into the borehole and to the surface. This method is not efficient because coal formations are generally not extremely porous and the majority of the methane is generally not found in the pores of the coal formation but is absorbed in or adsorbed to the coal. While methane can be produced from coal formations by this process, the production of methane is relatively slow.
In some coal formations, the natural permeability is sufficient to allow the removal of in situ water to permit the enhanced recovery of methane. In such formations, cleat systems developed during the coal bed diagenesis and burial history provide channel ways through which water and methane migrate to the production wells for removal. This removal of water or "de-watering" of the coal formations removes water from the channel ways and permits the flow of methane through the channel ways and to a production well at a greater rate.
Many coal formations do not have extensively developed cleat systems or have cleat systems which are not fully developed. These coal formations have very low permeability to water and gas and do not yield water or gas at significant rates. As a result, the water fills the cleats, and the recovery of methane from such coal formations is difficult or impossible at significant rates. Such low permeability water-containing coal formations may be either water saturated or less than fully water saturated. It appears that coal formations with better developed cleat systems may have been exposed to a diffusive oxidizing fluid of some type during the geologic past whereas coal formations with less developed cleat systems do not show evidence of past exposure to an oxidizing fluid.
Many formations containing carbonaceous materials in combination with inorganic materials show similar behavior. These are referred to as carbonaceous formations. Many such formations contain large quantities of methane, or other absorbed or adsorbed light hydrocarbons such as methane, but the methane is not readily recovered from such formations because the permeability and exposed surface area of the contained carbonaceous materials are too low to permit the efficient release of methane from the formation. The terms "absorbed" and "adsorbed" are used interchangeably in the discussion herein to refer to methane or other light hydrocarbons which are retained in or on the surfaces of the carbonaceous materials and the methane or other light hydrocarbons which are retained in or on the surfaces of the clay-mineral materials which are present in the carbonaceous formations.
Accordingly, continuing efforts have been directed to the development of methods for replicating the effects of the conditions which formed the better developed cleat systems in coal formations and increasing the production rate of methane from carbonaceous formations.